The objective of this research was to study two thermophilic bacteria for hydrogen and ethanol production from various types of sugars and complex lignocellulosic biomass. Strain AK14 is a moderate thermophilic bacterium belonging to the genus Clostridium. The bacterium performs the classical acetate-butyrate fermentation pattern with high hydrogen yields. Various physiological experiments e.g. the effect of partial pressure of hydrogen and initial substrate concentrations as well as growth on various carbon substrates, both sugars as well as hydrolysates from lignocellulosic biomass were investigated in detail for the strain. This work has been published in Icelandic Agricultural Sciences and is presented in this thesis as Manuscript I (Chapter 5) with the allowance of the editorial board of the journal. The other strain studied is Thermoanaerobacterium strain AK17.This strain has been intensively investigated in recent years because of its ethanol production capacity on carbohydrates. The present investigation studies the effects of various environmental factors (different concentrations of hydrolysates, enzymes and acid/base) on ethanol production capacity using simple sugars and various lignocellulosic biomass. Results are presented in Manuscript II (Chapter 6).